A conventional apparatus for controlling the air-fuel ratio of an internal combustion engine is disclosed in, for example, Japanese Unexamined Patent Publication No. 1-134749.
This disclosure will be roughly explained. An intake air quantity Q to the engine and an engine rotational speed N are detected to calculate a basic fuel supply quantity Tp (=K.Q/N, with K as a constant) for an intake air quantity supplied to a cylinder, the basic fuel supply quantity Tp is corrected according to an engine temperature, etc., and a feedback correction is carried out with an air-fuel ratio feedback correction coefficient (an air-fuel ratio correction quantity). This coefficient is based on a signal provided by an air-fuel ratio sensor (an oxygen sensor) for detecting the air-fuel ratio of an air-fuel mixture according to an oxygen concentration is an exhaust. The corrected quantity is further corrected according to a battery voltage, etc., to finally set a fuel supply quantity Ti.
A driving pulse signal having a pulse width corresponding to the fuel supply quantity Ti is provided to a fuel injection valve at a predetermined timing, to inject a predetermined quantity of fuel to the engine.
The air-fuel ratio feedback correction carried out according to the signal from the air-fuel ratio sensor is used to obtain a target air-fuel ratio (a theoretical air-fuel ratio). This feedback correction is carried out in an exhaust system because the converting efficiency (purifying efficiency) of an exhaust purifying catalytic converter (a three-way catalytic converter) for purifying an exhaust by oxidizing CO and HC (hydrocarbon) and reducing NOx contained in the exhaust is set to effectively function with a combustion based on the theoretical air-fuel ratio.
An electromotive force (an output voltage) of the air-fuel ratio sensor suddenly changes around the theoretical air-fuel ratio, and thus the output voltage V0 of the air-fuel ratio sensor is compared with a reference voltage (a slice level) SL corresponding to the theoretical air-fuel ratio, to determine whether the present air-fuel ratio is rich or lean with respect to the theoretical air-fuel ratio. If the air-fuel ratio is lean (rich), an air-fuel ratio feedback correction coefficient ALPP by which the basic fuel supply quantity Tp is multiplied is increased (decreased) by a large proportional portion P at the first time of a change to the lean (rich) side, and thereafter, is gradually increased (decreased) by a predetermined integral portion I. Accordingly, the fuel supply quantity Ti is increased (decreased) to obtain the target air-fuel ratio (the theoretical air-fuel ratio). The proportional portion may be omitted, and the air-fuel ratio feedback correction coefficient ALPP may be integrally set.
In this conventional air-fuel ratio feedback control apparatus, one air-fuel ratio sensor is arranged at a collecting portion of an exhaust manifold adjacent to a combustion chamber, to improve the responsiveness of the sensor. Since the temperature of an exhaust at this location is high, the properties of the air-fuel ratio sensor are deteriorated by heat. Also at this location, exhausts from respective cylinders are not sufficiently mixed, and thus it is difficult to detect a mean air-fuel ratio of all cylinders. This may lower the accuracy of detecting and controlling the air-fuel ratio.
To solve these problems, it has been proposed to arrange another air-fuel ratio sensor downstream of the exhaust purifying catalytic converter and carry out an air-fuel ratio feedback control according to the two air-fuel ratio sensors (Japanese Unexamined Patent Publication No. 61-237852).
Although the downstream air-fuel ratio sensor is not advantageous in terms of responsiveness, due to its distance from the combustion chamber, it is less affected by an imbalance of exhaust components (CO, HC, NOx, CO.sub.2, etc.,) on the downstream side of the exhaust purifying catalytic converter, and therefore, its characteristics are less affected by toxic components contained in an exhaust. Accordingly, the downstream air-fuel ratio sensor can detect a mean air-fuel ratio of all cylinders and provide more accurate and stabilized data than the upstream air-fuel ratio sensor.
Data provided by the two air-fuel ratio sensors are processed as mentioned above to provide two air-fuel ratio feedback correction coefficients. The two coefficients may be combined to accurately carry out the air-fuel ratio feedback control. Alternatively, the downstream air-fuel ratio sensor may be used to correct a control constant (a proportional portion or an integral portion) applied to the air-fuel ratio feedback correction coefficient set by the upstream air-fuel ratio sensor, or to correct a comparison voltage or a delay time related to the output voltage of the upstream air-fuel ratio sensor, to thereby compensate a fluctuation in the output voltage of the upstream air-fuel ratio sensor and accurately carry out the air-fuel ratio feedback control.
During a transient operation (an acceleration or deceleration operation), however, a response delay in the air-fuel ratio feedback control by the upstream air-fuel ratio sensor causes a large fluctuation in an air-fuel ratio. If the air-fuel ratio feedback control by the downstream air-fuel ratio sensor is carried out during this period, the air-fuel ratio will be over-corrected. During an acceleration, for example, the air-fuel ratio feedback control by the downstream air-fuel ratio sensor over-corrects the air-fuel ratio toward a rich side. As a result, after the acceleration, it takes a long time to restore the target air-fuel ratio, and in the worst case, the air-fuel ratio is widely diverged to thereby cause a deterioration of the fuel consumption, exhaust quality, and output of the engine.
Therefore, to avoid this, the transient operation is detected by determining whether or not a throttle valve is completely closed, or whether or not a rate of change of any one of a throttle valve opening, intake air quantity, intake air pressure, engine speed, and vehicle speed is greater than a predetermined value. If it is determined to be a transient operation, the air-fuel ratio feedback control based on the downstream air-fuel ratio sensor is stopped, to prevent an over-correction.
This transient operation determining technique for stopping the air-fuel ratio feedback control by the downstream air-fuel ratio sensor is effective if the degree of transience is large, but this technique demonstrates a poor accuracy and long delay time when the degree of transience is so low that it is barely sufficient to invert the air-fuel ratio feedback correction coefficient. In this case, this technique cannot prevent an over-correction of the air-fuel ratio.
To solve the problems of the prior arts, an object of the invention is to start and stop the air-fuel ratio feedback control of the downstream air-fuel ratio sensor by monitoring the output of the upstream air-fuel ratio sensor, and prevent an over-correction of air-fuel ratio during a transient operation.
Another object of the invention is to properly control an air-fuel ratio not only during a steady operation but also during a transient operation.
Still another object of the invention is to properly control an air-fuel ratio and reduce polluting exhausts such as CO, HC, and NOx.
Still another object of the invention is to properly control an air-fuel ratio and maintain good transient operation characteristics.